Ageing studies with micro-strip gas chambers

Micro-strip gas chambers (MSGCs) exposed for extended time to high radiation fluxes have been often found to suffer permanent deterioration of performance and modification of gain in the irradiated area, due to the deposition of thin insulating layers on the electrodes and/or substrate. The rate of ageing appears to be extremely sensitive to the presence of trace impurities either in the gas or released by the materials used for the detector assembly or in the gas mixing set-up.We show results of lifetime studies under sustained irradiation of MSGC plates manufactured on substrata with different bulk resistivity, 109 to 1015ohmcm, and with gold and chromium strips. They have been operated in several containers and assemblies. Under optimal experimental conditions, detectors have collected more than 100mCcm-1 without detectable decrease of gain, equivalent to many years of MSGCs operation in the Large Hadron Collider at CERN. The dependence of the rate of ageing on the cleanliness conditions and on the material of the strips or the nature of the substrate has been studied. (orig.)